Change tested Python versions to 3.8, 3.9 and 3.10

[pnuu]

Drop Python 3.7 from the tested Python versions and add Python 3.10.

[codecov]

Codecov Report

Merging #66 (6c185a0) into main (8d1fe8f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #66   +/-   ##
=======================================
  Coverage   46.49%   46.49%           
=======================================
  Files          15       15           
  Lines        2540     2540           
=======================================
  Hits         1181     1181           
  Misses       1359     1359           

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